lag behind phr. 落后；拖欠。
lag：v. 缓缓而行；落后于；加上外套。n. 时间间隔；滞后。
 
【例句】
Several players began to lag behind even at the beginning of the race.
比赛刚开始进行就有几名选手落在了后面。

If your payments of rent lag behind, you will be asked to leave.
你要是不能按时付房租，就请你搬出去。